1. Field of the Invention
The present invention relates to the field of industrial burners and in particular to radiant wall burners which operate to heat the surrounding portions of a wall of a furnace or the like, which often consist of a burner tile, and these heated surrounding portions then distribute heat by radiation in the furnace. Even more particularly, the invention relates to methodology and apparatus whereby the efficiency and capacity and NOx reduction capabilities of radiant burners is enhanced.
2. The State of the Prior Art
Reduction and/or abatement of NOx in radiant burners has always been a desirable aim. Moreover, it has always been a desirable aim in the industry to increase the heat production of known burners which use a primary premix produced by inducing a flow of air with fluid fuel but previous burners have not been capable of producing fuel-air premixes containing less than about 80% of the total fuel. Such premixes combust at high temperatures resulting in excessive production of NOx and other contaminants. Moreover, the amount of secondary fuel available for other purposes such as carrying flue gas into the flame has been extremely limited because the primary fuel-air premix includes the bulk of the fuel needed for combustion. Accordingly the industry has needed means for improving the efficiency of burners for radiant burner applications such that the primary pre-mix is leaner in fuel whereby a large mass of air is available during the initial combustion to reduce the combustion temperature and a large amount of secondary fuel is available for circulating in the furnace space away from the flame so as to premix with a large amount of flue gas to further reduce combustion temperatures. The industry has also needed radiant burners with greater heat production capacities.
The present invention alleviates the problems discussed above and enhances radiant burner installations by providing a high capacity, low NOx radiant wall burner assembly wherein the primary fuel-air premix has a much higher air content and a correspondingly much lower fuel content than previously thought possible by those skilled in the art. The burner of the invention is also capable of generating greater amounts of heat than previously known burners. In accordance with the concepts and principles of the invention, a high capacity radiant burner is provided which includes a burner tube structure comprising an elongated burner conduit having spaced inlet and outlet ends. The conduit is adapted and arranged for directing a fuel lean gaseous mixture comprising a portion of the total fluid fuel to be combusted and oxygen therealong from the inlet end to the outlet end. A main burner nozzle is provided at the outlet end of the conduit, and such burner nozzle has a central axis, a wall extending around a centrally located chamber therein, and a downstream end spaced from the outlet end of the conduit. The main burner nozzle is arranged and adapted for receiving the fuel lean fuel-air mixture from the conduit in the chamber and redirecting the same without substantial recirculation and with minimal pressure drop through a plurality of apertures in the wall and into a combustion zone in a direction transverse to the axis and at a velocity which is greater than the flame speed of the gaseous mixture. The apertures are distributed around the wall, whereby the fuel-air mixture directed into the combustion zone through the apertures is generally in the form of a round flat pattern which is detached from the nozzle, surrounds the wall and extends outwardly across a radiant surface of a burner tile. Ideally, the fuel lean gaseous mixture includes all of the oxygen needed for combusting the total fuel delivered to the furnace.
The burner of the invention also includes an elongated fuel tube that extends in a direction generally parallel to the axis of the nozzle. The fuel tube has a downstream end portion and a secondary fuel nozzle including at least one secondary fuel port is positioned on the downstream end portion of the fuel tube. Each secondary fuel port is located and arranged so as to deliver secondary fuel to a location in the furnace which is on the opposite side of the round flat pattern from the radiant surface and is sufficiently remote from the combustion zone to permit the same to become intermixed with flue gases before entering the combustion zone.
In accordance with the invention, the elongated fuel tube may be located externally of the main fuel nozzle and each secondary fuel port may be located and arranged so as to deliver secondary fuel at a velocity and in a direction such that at least a portion of the secondary fuel pierces the pattern to reach the proper location described above. Alternatively, the elongated fuel tube may extend through the main fuel nozzle and protrude through the downstream end thereof to deliver the secondary fuel directly to the location which is on the opposite side of the fuel-air pattern from the radiant surface.
Preferably, the burner tube structure may comprise a venturi tube which uses a flow of the gaseous fuel to induce a flow of air, whereby to create the fuel lean fuel-air mixture. Ideally, the mixture may comprise a mixture of a gaseous fuel and air.
In another form of the invention, the burner tube structure may comprise a plurality of venturi tubes arranged for parallel flow, each of the venturis being adapted and arranged to use a flow of the gaseous fuel to induce a flow of air, whereby to generate the mixture as an ultra fuel lean mixture of fuel and air.
In a more specific sense, the high capacity, low NOx radiant wall burner according to the invention may include an elongated nozzle arrangement adapted for installation in a central passageway of a refractory burner tile inserted in a wall of a furnace adjacent a combustion zone. The tile may preferably have a radiant surface surrounding the passageway and located adjacent the combustion zone. The nozzle arrangement may include an elongated burner tube including an elongated downstream portion configured to extend through the passageway and an elongated upstream portion, such portions may have respective centrally disposed, longitudinally extending axes. The nozzle arrangement may also include a fuel-air mixture supply system providing a source of a fuel lean combustible fuel-air mixture for introduction into the burner tube, an upstream end of the upstream portion of the burner tube being connected in fluid communication with the fuel supply system for receiving the fuel lean combustible fuel-air mixture, the burner tube providing a conduit for flow of the fuel lean combustible fuel-air mixture therealong from the upstream end to a downstream end of the downstream portion of the burner tube.
The nozzle arrangement of the invention may further include a main nozzle positioned at the downstream end of the downstream portion of the burner tube adjacent the radiant surface, the main nozzle having an internal chamber that is in fluid communication with the downstream end of the downstream portion of the burner tube for receiving the fuel lean combustible fuel-air mixture flowing along the tube. The main nozzle is arranged and configured to redirect the fuel-air mixture in the chamber and cause it to flow without substantial recirculation in a direction radially outwardly relative to the axis of the downstream portion of the burner tube, into the combustion zone, and generally across the radiant surface. The main nozzle has a wall extending around the chamber and a series of radially extending openings in the wall. The openings are arranged and configured to dispense the combustible fuel-air mixture in a radial direction at an initial velocity which exceeds the flame speed of the mixture and in a circular pattern which essentially surrounds the nozzle in a radial direction, whereby a detached round flame is created when the mixture is combusting. Finally, the burner arrangement may desirably include a secondary fuel nozzle system including an elongated fuel tube extending longitudinally of the downstream portion of the burner tube and having at least one fuel gas port disposed and arranged to direct a flow of secondary fuel to a location in the furnace on an opposite side of the combustion zone from the radiant surface. The secondary fuel constitutes a substantial portion of the total fuel provided to the combustion zone by the fuel-air mixture supply system and the secondary fuel nozzle system.
In accordance with a highly preferred form of the invention, the fuel-air supply system of the burner may comprise an ejector including a fuel inlet connectable to a source of pressurized fluid fuel, a fluid fuel spud connected in fluid communication with the inlet and positioned for ejecting fluid fuel through a space in fluid communication with a source of air, and a generally bell-shaped fitting mounted at the upstream end of the upstream portion of the burner tube. The bell-shaped fitting has a mouth positioned for receiving the ejected fluid fuel and air carried along with it and directing the same into the upstream end of the burner tube.
In one form of the invention, the axes of the portions of the burner tube may be superimposed whereby the burner tube is essentially straight. Thus, the main nozzle, the burner tube and the ejector are in essential alignment along the superimposed axes. In an alternative form of the invention, the axis of the upstream portion may be disposed at an angle relative to the axis of the downstream portion, whereby the main nozzle and the downstream portion of the burner tube are disposed in essential alignment along the axis of the downstream portion, and the ejector and the upstream portion of the burner tube are disposed in essential alignment along the axis of the upstream portion.
In one form of the invention, the elongated fuel tube may be located outside the main nozzle. Preferably, in this form of the invention, the secondary fuel nozzle system may include a plurality of elongated fuel tubes located outside the main nozzle. Desirably, the ports of the secondary fuel tubes are each configured and positioned to cause at least a portion of the secondary fuel to pierce the fuel-air mixture pattern and reach the desired location in the furnace without combusting.
In another form of the invention, the main nozzle may includes an end cap having a hole in it, and wherein the fuel tube extends through the chamber and a downstream portion thereof protrudes through the hole. A port in the downstream portion of the fuel tube is positioned adjacent the desired location in the furnace. Desirably, a plurality of ports may be provided in the downstream portion of the fuel tube and the location in the furnace may surround the downstream portion of the fuel tube.
In accordance with the concepts and principles of the invention, the radiant surface may be either essentially flat or cup-shaped. Desirably, the end cap may be convex relative to the chamber.
In another form of the invention, where the secondary fuel nozzle extends through the main nozzle and an eductor is used to premix the primary fuel-air mixture, the secondary fuel system may desirably be arranged to bypass the eductor. This may be done as discussed above by arranging the axes of the upstream and downstream portions of the burner tube at an angle. Alternatively, the secondary fuel system may include a segment of tubing which extends laterally through a wall of the downstream portion of the burner tube, such segment being connected in fluid communication with an upstream end of the fuel tube.
In a highly preferred form of the invention, the openings in the nozzle wall may desirably comprise elongated slots which extend in a direction that is essentially parallel to the axis of the downstream portion of the burner tube. Preferably, the wall of the nozzle may comprise a series of circumferentially spaced bars presenting the slots therebetween, the bars having rounded surfaces adjacent the chamber to inhibit the formation of recirculation zones in the chamber. Ideally, the burner may include an internal baffle having a generally bell-shaped downstream portion located in the chamber. The bell-shaped portion may have an outer, circumferentially extending edge disposed adjacent the wall. Additionally, the slots may have an upstream end and a downstream end, and the outer edge of the bell-shaped portion may be located closer to the upstream end of the slot than to the downstream end of the slot. Ideally, the outer edge of the bell-shaped portion may be located approximately one-fourth of the distance from the upstream end of the slot to the downstream end of the slot. Furthermore, the slots may desirably have upstream end surfaces that slope in a direction of fluid flow to inhibit the formation of recirculation zones in the chamber.
In a preferred form of the invention, the fuel-air mixture supply system and the secondary fuel system may be arranged such that the amount of the secondary fuel constitutes more than about 20%, desirably at least about 30% and ideally at least about 50 to 60% of the total fuel provided to the combustion zone. In a further preferred form of the invention, the relationship between the velocity that the primary fuel-air mixture leaves the slots and the flame speed of the mixture is such that the upstream extremity of the detached flame is positioned between about 1 inch and 3 inches from the nozzle to make sure that the radiant tile is heated evenly.
In accordance with another preferred aspect of the invention, when the axes of the upstream and downstream portions of the burner tube are disposed at an angle, the burner tube may desirably include a curved portion which interconnects the downstream and upstream portions thereof, and the secondary fuel system may include a segment of tubing which extends through a wall of the curved portion of the burner tube. This segment of tubing is connected in fluid communication with an upstream end of the fuel tube. Ideally, the arrangement is such that the segment of tubing and the fuel tube extend essentially along the axis of the downstream portion of the burner tube and the eductor is offset at an angle. With this arrangement, the eductor for the primary fuel-air mixture is bypassed by the secondary fuel system, and the overall longitudinal dimensions of the burner are reduced.
The invention further provides a method for operating a high capacity, low NOx radiant wall burner. The method comprises (1) delivering a flow of a fuel lean combustible mixture comprising a portion of the total fuel to be combusted and air in a radial direction from an elongated nozzle having a central axis to a combustion zone surrounding the nozzle in the form of a round flat pattern which surrounds the wall and at a composition where the flame speed of the mixture is lower than the velocity of the mixture as the latter exits the nozzle, the combustion zone being adjacent a radiant face of a burner tile; (2) igniting the mixture to create a round flat detached flame which surrounds the nozzle in a radial direction and is located adjacent the radiant face; and (3) providing a supply of secondary fuel at a location on the opposite side of the flame from the radiant face and spaced far enough away from the flame so that the secondary fuel becomes intermixed with flue gas before it enters the flame.
More specifically, the method may desirably comprise (1) providing a fuel lean combustible fuel-air mixture; (2) causing the fuel-air mixture to flow outwardly from a main nozzle, into the combustion zone and generally across the radiant surface in a circular pattern which essentially surrounds the main nozzle in a radial direction; (3) causing the fuel-air mixture to flow outwardly from the main nozzle at an initial velocity which exceeds the flame speed of the mixture, whereby a detached round flame is created when the mixture is combusting; and (4) providing a secondary fuel at a location in the furnace on an opposite side of the zone from the radiant surface, the secondary fuel constituting a substantial portion of the total fuel provided to the combustion zone by the fuel-air mixture supply system and the secondary fuel nozzle system.
In accordance with the invention, the secondary fuel desirably constitutes more than about 20%, preferably constitutes at least about 30% and ideally constitutes at least about 50 to 60% of the total fuel provided to the combustion zone.
In one form of the invention, the secondary fuel is provided at the location on the opposite side of the primary fuel-air pattern using a secondary fuel nozzle which extends through the main nozzle. Alternatively, the secondary fuel is provided at the location using a secondary fuel nozzle which emits a jet of fuel that pierces the pattern without combusting.